This invention relates generally to the isomerization of hydrocarbons. More specifically, the invention involves an isomerization zone and an isomerized product fractionation zone in which a stabilized effluent stream from the isomerization zone is separated into high octane product streams and low octane product streams by means of fractional distillation and by making use of a dividing wall column and a non-divided column. The stabilized isomerization zone effluent is generally comprised of hydrocarbons containing between 5 and 8 carbon atoms per molecule.
Isomerization is an important process used in the petroleum industry to increase the research octane number (RON) of light naphtha feeds. In current practice, the naphtha (C5-C10 fraction) obtained from atmospheric distillation of petroleum is separated by means of fractional distillation into light naphtha (C5-C6 fraction or C5-C7 fraction depending on desired volume of light naphtha) and heavy naphtha (C7-C10 fraction or C8-C10 fraction depending on desired volume of light naphtha). The light naphtha is generally sent to an isomerization process unit and the heavy naphtha is generally sent to a catalytic reforming process unit. In both the isomerization process unit and the catalytic reforming process unit, the RON values of the respective naphtha fractions are improved. High RON values are a desired characteristic for naphtha streams that are sent to the gasoline pool because gasoline spark ignition engines perform better and can achieve greater fuel efficiency with higher RON gasoline.
The product streams from isomerization processes (isomerate), unlike the product streams from catalytic reforming processes (reformate) are virtually free of aromatic compounds. Low aromatic concentrations are a desired characteristic for naphtha streams that are sent to the gasoline pool because of increasingly stringent specifications for aromatics in gasoline. As a result of the increasingly stringent specifications for aromatics in gasoline, there has been growing interest in the petroleum industry in processing a greater volume of light naphtha in isomerization process units.
The present invention relates in particular to C5-C7 fraction light naphtha feeds to isomerization units that are rich in C5-C8 molecules. The C5-C7 fraction is generally produced through fractionation of full range naphtha in such a manner that the majority of the C8 molecules found in the full range naphtha are excluded from the C5-C7 fraction. However, a small percentage of the C8 molecules from the full range naphtha will be included in the C5-C7 fraction as a result of overlap that is characteristic of distillation processes. Therefore, the term “C5-C7 fraction” will be used herein to designate a fraction that contains C5-C8 molecules but in practice is materially a C5-C7 fraction.
Several processes for isomerizing C5-C7 fraction light naphtha feeds are described in the patent literature. Two such examples of recent patents are U.S. Pat. No. 6,338,791 and U.S. Pat. No. 7,429,685. U.S. Pat. No. 6,338,791 describes various process flow schemes to isomerize a C5-C7 fraction and separate the isomerization reactor effluent into high octane streams and low octane streams. U.S. Pat. No. 7,429,685 describes various process flow schemes in which the C5-C7 fraction is first separated into a C5-C6 fraction and a C7 fraction before passing the two fractions independently to two parallel isomerization reactors, from where the isomerization reactor effluents are separated into high octane streams and low octane streams. U.S. Pat. No. 7,429,685 describes several separation configurations; in one configuration the reactor effluents are separated independently, and in another configuration the reactor effluents are combined and separated.
The separation of the isomerate reactor effluent in isomerization processes is critical to achieving the desired RON target for the isomerate product. In order to maximize the isomerate product RON, it is desirable to separate the isomerization reactor effluent into different molecular structural classes. In general, multibranched paraffins (paraffins having two or more branches) have higher RON values than straight chain and single branched compounds. It is desirable, therefore, to separate the high octane multibranched compounds (as well as high octane isopentane) as isomerate product and recycle lower octane straight chain and single branched paraffins to the reactor feed. It is generally not desirable to recycle multibranched paraffins to the reactor feed because doing so would result in the conversion of a portion of the high octane multibranched paraffins into lower octane straight chain and single branched paraffins in the isomerization reactor.
Several methods that have been utilized to achieve the desired separation between high octane components and low octane components in isomerization reactor effluents in applications with C5-C6 fraction light naphtha feeds are described in Domergue, B., and Watripont, L. World Refining, May 2000, p. 26-30.
None of the methods outlined in the Domergue and Watripont article make use of a dividing wall column to separate high octane components and low octane components in isomerization reactor effluents. In general, a significant improvement in the efficiency of separation can be achieved through separations that are performed in dividing wall columns compared with the use of multiple non-divided columns to perform the same separations because of the superior thermal efficiency of dividing wall columns. An alternate scheme for achieving the desired separation between high octane components and low octane components in isomerization reactor effluents in applications with C5-C6 fraction light naphtha feeds using a combination of adsorption and a dividing wall column is described in U.S. Pat. No. 6,395,951.
Separating isomerization reactor effluents in applications with C5-C7 fraction light naphtha feeds is significantly more complicated than in applications with C5-C6 fraction light naphtha feeds, especially when high values of isomerate RON are required. Schemes that require the use of a deisohexanizer to recover and recycle methylpentane compounds (single branch C6 paraffins) in applications with C5-C6 fraction light naphtha feeds increase in complexity in applications with C5-C7 fraction light naphtha feeds and require the use of a deisohexanizer and a deisoheptanizer to achieve a high isomerate product RON. Deisohexanizer columns are generally large, costly to construct and install, and consume large amounts of reboiler energy because of the difficult separation between close boiling high octane multibranched C6 paraffins such as dimethylbutanes and low octane single branched C6 paraffins such as methylpentanes. Deisoheptanizer columns present the same drawbacks as deisohexanizer columns because of the difficult separation between close boiling high octane multibranched C7 paraffins such as dimethylpentanes and low octane single branched C7 paraffins such as methylhexanes.
An example of a conventional method for separating a combined isomerization zone effluent mixture by fractionation into high octane and low octane streams is shown in FIG. 1. The charge to the isomerization process is sent via line 12 to charge fractionation zone 20. The charge fractionation zone may produce one or more primary feeds to the isomerization zone. Two primary feeds to the isomerization zone are shown in the example in FIG. 1. The two primary feeds are conducted from the charge fractionation zone 20 to isomerization zone 22 via lines 14 and 16. In the example shown in FIG. 1, the stream that is conducted via line 14 represents a C5-C6 fraction and the stream that is conducted via line 16 represents a C7 fraction. Two recycle streams from the isomerized product fractionation zone are also sent to the isomerization zone. A C6 rich recycle stream is conducted via line 42 and mixed with the C5-C6 fraction primary feed to create a combined C5-C6 isomerization zone feed stream which is conducted via line 18 to isomerization zone 22. A C7 rich recycle stream is conducted via line 34 and mixed with the C7 fraction primary feed to create a combined C7 isomerization zone feed stream which is conducted via line 24 to isomerization zone 22. Two reactor effluent streams exit the isomerization zone via lines 26 and 28 and are sent to two independent stabilizers (not shown in FIG. 1) to remove butane and lighter hydrocarbons. A stabilized isomerized product is removed from each of the two stabilizers.
The stabilized isomerized product corresponding to a C5-C6 fraction is sent via line 26 to deisohexanizer column 38. The C5-C6 fraction isomerized product is separated into three streams in the deisohexanizer column: a first high octane stream comprising the major portion of hydrocarbons containing 5 carbon atoms and paraffins containing 6 carbon atoms with at least two branches is removed from the first end of the column via line 40, a low octane stream comprising the major portion of normal hexane and paraffins containing 6 carbon atoms and a single branch is removed as a side stream from an intermediate point in the column via line 42, and a second high octane stream comprising the major portion of hydrocarbons containing at least 7 carbon atoms is removed from the second end of the column via line 44 (note that this stream may optionally be recycled to the isomerization zone or to the charge fractionation zone). The term “first end of the column” is used herein to refer to the overhead distillate system (at the top) of the column and the term “second end of the column” is used herein to refer to the bottom of the column.
The stabilized isomerized product corresponding to a C7 fraction is sent via line 28 to deisoheptanizer column 30. The C7 fraction isomerized product is separated into three streams in the deisoheptanizer column: a first high octane stream comprising major portion of hydrocarbons containing 6 carbon atoms and paraffins containing 7 carbon atoms with at least two branches is removed from the first end of the column via line 32, a low octane stream comprising the major portion of normal heptane and paraffins containing 7 carbon atoms and a single branch is removed as a side stream from an intermediate point in the column via line 34 and a second high octane stream comprising the major portion of hydrocarbons containing at least 8 carbon atoms is removed from the second end of the column via line 36.
A conventional method for separating isomerization zone effluent streams as shown in FIG. 1 is energy inefficient because high energy inputs are required for each of the two columns to achieve the required separation of high octane and low octane streams. High energy inputs are required for both columns because each of the two columns are designed to separate close boiling high octane multibranched paraffins and low octane single branched paraffins.
The separation scheme presented in U.S. Pat. No. 6,395,951 employs a unique configuration consisting of an adsorptive separation zone followed by a dividing wall fractionation zone to separate isomerization zone effluent streams into high octane and low octane fractions. Low octane straight chain paraffins such as normal hexane are removed in the absorptive separation zone for recycle to the isomerization zone and a dividing wall column in the dividing wall fractionation zone separates low octane single branched C6 paraffins from high octane multibranched C6 paraffins and from a high octane C6-C7 bottoms stream. The separation in the dividing wall column for this design is notably different than separations which are made in typical deisohexanizer column designs in that the majority of (high octane) methylcyclopentane is intentionally removed as part of the high octane C6-C7 bottoms stream. This contrasts with a typical deisohexanizer design that does not have an adsorptive separation section to remove low octane straight chain paraffins. Normal hexane (a straight chain molecule) is present in the feed to a typical deisohexanizer design that does not have an adsorptive separation section to remove low octane straight chain paraffins, and because normal hexane has a very low octane value, it is desirable to include as much normal hexane as possible in the low octane fraction containing low octane paraffins with a single branch so that the normal hexane can be recycled to the isomerization zone for conversion to isomerized products. Normal hexane and methylcyclohexane are close boiling molecules, and as a result of including the majority of normal hexane in the low octane fraction containing low octane C6 paraffins with a single branch, the majority of methylcyclopentane is also removed from a typical deisohexanizer column in the low octane stream containing normal hexane and C6 paraffins with a single branch. In effect, the methods described in U.S. Pat. No. 6,395,951 use a dividing wall column to create high octane and low octane fractions that have different compositions with respect to methylcyclopentane than typical deisohexanizer separations.
The unique manner in which the method outlined in U.S. Pat. No. 6,395,951 using absorptive separation in conjunction with fractional distillation means that in order to apply this method to separate isomerization zone effluent streams in applications with C5-C7 fraction light naphtha feeds, at least two dividing wall columns would be required; one to segregate low octane single branched C6 paraffins for recycle to the isomerization zone and a second to segregate low octane single branched C7 paraffins for recycle to the isomerization zone. Each of the two dividing wall columns would require high energy input to achieve the desired separation between close boiling low octane single branched paraffins and high octane multibranched paraffins, which makes the approach described in U.S. Pat. No. 6,395,951 poorly suited for applications with C5-C7 fraction light naphtha feeds.
The use of a fractional distillation scheme involving a dividing wall column and a non-divided column in the present invention to separate a combined isomerization reactor effluent in a process with a C5-C7 fraction light naphtha feed provides significant advantages versus methods that are currently publically known because the energy intensive separations between close boiling high octane multibranched paraffins and low octane single branched paraffins are combined into a single dividing wall column, thereby reducing the amount of distillation energy input associated with conventional fractionation techniques.